1. Field of the Invention
The present invention relates to an active noise elimination apparatus, more particularly, an active noise control system with a detouring sound apparatus which precisely eliminates noise by taking into consideration the fact that the sound generated from a sound generation device, for eliminating noise from a noise source, indirectly reaches the noise source.
The environment has become a major social issue in recent years. Noise is also becoming a social issue because it is detrimental to the living and working environments and has an adverse effect on health. Recently, so-called "active noise eliminators" which not only eliminate noise by absorbing it, but also eliminate noise by generating sound waves having the same amplitude, but an opposite phase to the waveforms of the noise, to thereby cancel the noise have attracted increasing attention. There is a strong demand for an active noise eliminator which can be applied to all types of apparatuses and equipments generating noise, such as electrical home appliances and computer systems, and which can eliminate noise efficiently and economically.
2. Description of the Related Art
In a cooling and silencing control system for a large high-speed computer system, which cools the computer system by blowing cool air, cool air is blown from a cooling apparatus below a free-access floor. A cooling control system sucks this cooling air into a duct by a fan and exhausts it through the duct. In this way, the heat generated from heat sources, such as the printed circuit boards of the computer, is guided to and exhausted through the duct. The cooling control system controls the cooling by changing the rotational speed of the fan in accordance with the temperature. In the case of small computers, room temperature air is caused to flow through the heat sources such as the printed circuit boards instead of cool air. In either case, the active noise cancelling controller (ANCC) drives a sound generation device, such as a speaker, based on the noise from the fan (fan noise) received by a sensor microphone, and fan noise received by an error microphone remaining after noise cancellation (residual noise) so as to generate sound waves having the same amplitude but an opposite phase to the fan noise. The fan noise is cancelled out by the sound generated from the speaker (speaker sound) to thus actively eliminate the fan noise.
The fan noise received by the sensor microphone, that is, the microphone disposed in the proximity of the cooling fan (noise source) for cooling the printed circuit boards, etc, is converted from an analog to a digital signal by an analog/digital converter (A/D converter). The signal is then and input to an adaptive type finite impulse response (FIR) filter, giving a transmission coefficient simulating the physical propagation route of the sound through the duct. The output of this FIR filter is converted from a digital to an analog signal by a digital/analog converter (D/A converter). The speaker is driven by this signal so as to generate sound waves having the same amplitude as, but an opposite phase to, the noise generated by the fan. The fan noise is eliminated by being offset by this speaker sound.
The residual noise, which remains when noise cannot be completely eliminated by cancellation of the fan noise by the speaker sound, that is, the sound generated by the error of the result of simulation of the fan noise by the FIR filter (residual error), is received by the error microphone. This analog signal is converted to a digital error signal by the A/D converter. The filter coefficient (or tap coefficient) of the FIR filter is changed on the basis of this error signal so as to bring the residual error, that is, the residual noise, close to zero, and thus completely eliminate the noise generated by the fan.
The processing described above is generally executed within a sampling period t of the A/D converter connected to the sensor microphone and is repeated at intervals equal to the period t to eliminate the fan noise.
The fan noise elimination processing described above is based on the prior art, which does not take into consideration the detouring sound from the speaker to the sensor microphone. In practice, the speaker sound travels indirectly towards the fan, cancels the fan noise, and then is received by the sensor microphone. Accordingly, to efficiently eliminate the noise generated from the cooling system, processing which takes detouring sound into consideration is necessary. Processing for eliminating the influence of the detouring sound is required in the fan noise elimination processing described above.
According to the noise elimination system of the prior art, two FIR filters, that is, an FIR filter for the detouring sound processing and another FIR filter for the fan noise elimination processing, are provided in one processing unit (for example, a digital signal processor: DSP). The detouring sound processing can be executed by the former and then the fan noise elimination processing can be executed by the latter.
However, according to the prior art system described above, since the detouring sound processing and the fan noise elimination processing are executed in series, a long time (for example, about twice the sampling period t of the A/D converter for sampling the noise from the sensor microphone) is necessary for the noise elimination control, and the duct length must be increased (to about double, for example) so as to secure the necessary time. This is economically disadvantageous. To complete the noise elimination processing within the period t without increasing the duct length, a DSP having a higher operating speed and higher performance must be employed. This is neither economical nor efficient.